Radio communication systems typically use a receiver (e.g., a selective call receiver or "pager") that has at least one unique call address associated therewith. These radios receive and decode at least one specific address, then typically alert the user to the presence of incoming information and operate to present this information. Radio communication systems are an excellent vehicle for delivering voice, numeric, alphanumeric or coded information to a user.
In contemporary selective call receivers, selective call addresses and operating features are selected by an integrated circuit controller (e.g., a microprocessor or the like) that reads information from a non-volatile memory such as a code plug. The same basic integrated circuit controller may be used in a number of selective call receiver models ranging from a basic low tier model (e.g., tone only alerting) to a full featured high tier model (e.g., alphanumeric display and voice messaging). In the past, to establish tiering among these models, a common base integrated circuit layout was designed with a plurality of unique, optional metal mask layers, each representing different levels of functionality (e.g., more or less options). More modern configuration methods use the same basic layout with either zener diode (electrically severed) or laser trimmable programming or trim links. The links are initially continuous and are selectively severed, depending on the requirements of the integrated circuit.
With integration of the programming links comes a new problem. In order to sever these links, one must use either a high power, extremely accurate laser, or apply a high voltage bias across a selected link, thereby melting the link. Because of a large beam spot size in relation to the absolute device size and geometry in state of the art integrated circuits, laser trimming has become impractical. Electrically programming links using zener trims also has problems. Since the programing links are electrically connected to their respective circuits, there is no way to prevent the trim bias from being applied to the active and passive devices comprising the integrated circuitry. This imposes an undesirable and sometimes impossible design constraint in that the breakdown potentials of the devices interconnected to the links must exceed by a significant amount, the trim bias potential applied thereto. If this constraint is not met, damage or destruction of the circuitry will occur.
Consequently, what is needed is an apparatus for electrically programming metal mask options that will provide sufficient isolation of the programming bias potential from the integrated circuitry, thereby preventing any damage to the circuit devices.